staging: vt6656: Fix warning: unused variable vnt_frame_time

In commit 61bb7987
("staging: vt6656: vnt_get_rtscts_rsvtime_le replace with rts/cts duration.")
not quite all of the code was removed.

Remove unused vnt_frame_time variable.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/5096f399-03e7-77e1-b334-947aabc44d14@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>